1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus for implementing the same which are suitably applied to a scanning exposure apparatus for exposing a pattern on a mask onto a photosensitive substrate by synchronously scanning the mask and the photosensitive substrate.
More particularly, the present invention relates to an exposure method and apparatus to be used in a photolithography process for manufacturing a semiconductor device, a liquid crystal display device, a thin film magnetic head, etc.
2. Related Background Art
When a semiconductor device, a liquid crystal display device or a thin film magnetic head is to be manufactured by using a photo-lithography technology, a projection exposure apparatus which exposes a pattern of a photo-mask or a reticle (hereinafter collectively referred to as a reticle) onto a photosensitive substrate such as a wafer or a glass plate on which photo-resist is applied through a projection optical system has been used. Recently, as the size of a single chip pattern of the semiconductor device tends to increase, it is required to increase the exposure area on the photosensitive substrate so that a larger pattern of the reticle may be exposed.
In order to comply with the requirement of the increased area, a scanning projection exposure apparatus for exposing a pattern on a reticle onto a photosensitive substrate by synchronously scanning the reticle and the photosensitive substrate to an illumination area of rectangular, arcuate-or hexagonal shape (hereinafter collectively referred to as a slit illumination area) has been developed (U.S. Pat. Nos. 4,747,678, 4,924,257). In the past, as shown in U.S. Pat. No. 5,194,893, in order to define a slit illumination area on the reticle, a movable light shielding means (view field diaphragm) for determining the slit illumination area is arranged at a position which is conjugate with the reticle or in the vicinity of the reticle. The shape of the slit illumination area on the reticle and the shape of the slit illumination area on the photosensitive substrate are controlled by a design constant or an apparatus constant.
In such a prior art apparatus, the following two major problems were encountered.
In general, an illumination optical system of the projection exposure apparatus is designed to illuminate the reticle with a uniform illumination light (exposure light). Accordingly, in the scanning projection exposure apparatus, in order to assure that the uniformity in the illumination is attained on the photosensitive substrate at the time when the exposure is completed after the scanning of the reticle and the photosensitive substrate relative to the slit illumination area, the width of the slit illumination area along the scan direction must be uniform.
Assuming that X represents the scan direction to the slit illumination area and Y represents a non-scan direction perpendicular to the scan direction, if the width of the slit illumination area along the scan direction is not uniform, the parallelism of the slit illumination area 30 along the scan direction is bad as shown in FIG. 5A, or edges of the slit illumination area 31 along the scan direction include unevenness as shown in FIG. 6A. In the case of FIG. 5A, a distribution of the exposure intensity E along the non-scan direction (Y axis) measured on the photosensitive substrate gradually increases or decreases along the Y axis as shown in FIG. 5B. On the other hand, in the case of FIG. 6A, a distribution of the exposure intensity E along the non-scan direction (Y axis) measured on the photosensitive substrate varies irregularly along the Y axis as shown in FIG. 6B.
In the present projection exposure apparatus, a design rule of less than 0.5 .mu.m which is in a sub-micron area is used. It is reported that the uniformity of the exposure intensity required for the control of a line width in such an area is as small as .+-.1%. Accordingly, in order to attain a sufficient uniformity of the illumination in the scanning projection exposure apparatus, the reduced unevenness of the edge along the scan direction is required for a view field diaphragm (stop) for determining the slit illumination area, and the control of the motion in changing the width of the view field diaphragm along the scan direction while maintaining sufficient parallelism of the edge along the scan direction is required, as disclosed in Japanese Patent Application Laid-Open No. 4-196513. As a result, it is difficult to control the operation of the illumination area setting means while keeping the required precision if the view field diaphragm for defining the slit illumination area is varied in synchronism with the scan by a reason to be described later.
Further, as shown in FIG. 7, it is assumed that two circuit pattern areas 32A and 32B are arranged with a light shielding area of a width L1 therebetween on a reticle R, and the reticle R is scanned to a slit illumination area 33 of a width L2 along the scan direction. It is further assumed that the width L2 of the slit illumination area 33 is larger than the width L1 of the light shielding area between the circuit pattern areas 32A and 32B. In this case, if only the first circuit pattern area 32A of the reticle R is to be exposed onto the photosensitive substrate by the scanning exposure system, a portion of the pattern of the second circuit pattern area 32B is also transferred onto the photosensitive substrate.
In order to avoid the above problem, the width L1 of the light shielding area on the reticle R may be set to be sufficiently large but this would result in the reduction of the area of the circuit pattern area to be transferred. Alternatively, the width L2 of the slit illumination area 33 may be reduced in synchronism with the scan closely to the end of the exposure of the circuit pattern area 32A as shown in Japanese Laid-Open Patent application Ser. No. 4-196513, but this would complicate the control mechanism for the variable view field diaphragm.
In the prior art projection exposure apparatus, the illumination optical system is designed to illuminate the reticle with a uniform illumination. Assuming that I (mW/cm.sup.2) represents an illumination on a surface of the photo-sensitive substrate (image plane illumination), S (mJ/cm.sup.2) represents a desired exposure intensity (sensitivity of a photosensitive material on the photosensitive substrate), D (mm) is a width of slit illumination area on the surface of the photosensitive substrate along the scan direction, and v (mm/sec) is a scan velocity of the photosensitive substrate, a required exposure time t (sec) is given by: EQU t=S/I=D/v (1)
The exposure intensity S is entered by an operator and the illumination I is normally determined by the intensity of the available light source. Accordingly, in order to attain the exposure intensity S entered by the operator, it is necessary to determine the scan velocity v in accordance with the width D of the slit illumination area along the scan direction. If the exposure intensity S is so small that the scan velocity v exceeds a maximum scan velocity v.sub.max permitted to the apparatus, it is necessary to reduce the illumination I by dimmer means in the illumination optical system or reduce the width D of the slit illumination area along the scan direction. Assuming that M represents a projection magnification of the projection optical system, the scan velocity of the reticle is given by V/M (mm/sec).
FIGS. 11A to 11D show various examples of the areas corresponding to the slit illumination area on the photosensitive substrate. FIG. 11A shows an area 130 corresponding to a rectangular illumination area of a width D along the scan direction. FIG. 11B shows an area 131 corresponding to an arcuate illumination area of a width D along the scan direction. FIG. 11C shows an area 132 corresponding to a hexagonal illumination area of a width D along the scan direction as disclosed in Japanese Laid-Open Patent Application No. 46-34057, in which opposite ends 132a and 132b of the area 132 perpendicular to the scan direction (along the non-scan direction) overlap with the adjacent scan areas to assume advantageous shapes when they are scanned. FIG. 11D shows an area 133 corresponding to a diamond shaped illumination area of a width D along the scan direction as disclosed in Japanese Laid-Open Patent Application No. 53-25790, in which opposite ends 133a and 133b of the area 133 along the non-scan direction overlap with the adjacent scan areas to assume advantageous shapes when they are scanned.
However, since the prior art projection exposure apparatus is nor equipped with measurement means for the width D of the slit illumination area along the scan direction, it is difficult to expose to the photosensitive substrate with a proper exposure intensity if the actual width D along the scan direction deviates from a design value or the apparatus constant. Presently, reduction projection type exposure apparatuses (steppers) of a step-and-repeat system disclosed in e.g., U.S. Pat. Nos. 4,677,301 and 4,962,318 have been widely used. As illumination for exposure, emission lines (i-line and the like) from a mercury lamp, a KrF or ArF excimer laser or a higher harmonic such as of a metal vapor laser or a YAG laser is used.
In projection exposure apparatuses as disclosed in e.g., U.S. Pat. Nos. 4,712,910 and 4,884,101, a shutter is utilized to open and close the path of light from a light source thereby to control the amount of exposure. That is, the amount of exposure imparted to a wafer is controlled to an optimum value corresponding to the sensitivity of the photoresist of the wafer. Especially in projection type exposure apparatuses with pulsed laser light sources such as of an excimer laser or the like, as disclosed in, e.g., U.S. Pat. Nos. 4,970,546, 5,097,291 and 5,191,374, an amount of energy per pulse is set to a predetermined value thereby to control the amount of exposure.
Recently, as semiconductors become large in size and minute in structure, it is required to enlarge the image field of the projection optical system and to improve the resolution thereof. However, it is extremely difficult to obtain both the high resolution and the large image field in the projection optical system from the viewpoint of design and manufacture. Therefore, as disclosed in, e.g., U.S. Pat. Nos. 4,747,678, 4,924,257 and 5,194,893, scanning type projection exposure apparatuses are paid attention in which only a local area of a reticle is illuminated and the reticle and a wafer are shifted synchronously to expose the pattern of the reticle to the wafer. In such scanning type exposure apparatuses, even though the image field of a projection optical system is small, it is possible to expose a pattern with a large area to the wafer and to improve the resolution of the projection optical system comparatively easily.
However, if the conventional exposure control method is applied to such scanning type exposure apparatuses, the amount of exposure to the wafer cannot be controlled to an optimum value corresponding to the sensitivity of the photoresist. That is, in a scanning type exposure apparatus with a light source emitting continuous light such as of i-lines, even though only a time for opening a shutter is controlled as in U.S. Pat. No. 4,712,910, an optimum amount of exposure cannot be imparted to the wafer. Also, when the sensitivity of the photoresist is changed, the amount of exposure cannot be controlled properly in accordance with the change. Further, in a scanning type exposure apparatus with a light source emitting a light beam such as an excimer laser, etc., there is a chance that the number of light beams illuminating a wafer is different in various positions on the wafer in accordance with the relationship between the rate of movement of the wafer and the timing of emissions of light beams. Namely, there is a change that unevenness of the amount of light occurs.